Keystone correction circuit



Feb. 20, 1945. c. E. HALLMARK KEYSTONE CORRECTION CIRCUIT .l

Filed Apr'il 3o, i942 nNvENTo'R Kuijl-24 mm2/Om verting optical images into of the scanning pattern on scanning wave.

Patented'Feb. 20, 1945 2,369,824 KEYSTON E CORRECTION CIRCUIT Clyde E. Hallmark, Fort Wayne, Ind.,

assigner to Farnsworth Television and Radio Corporation, a corporation of Delaware Application April 30, 1942, Serial No. 441,205

4 Claims.

This invention relates generally to a scanning system for television systems and more particularly to scanning circuits for correcting keystone distortion in such systems. This invention is a modification of that disclosed in applicants copending application Serial No. 437,118 dated April l, 1942, for Keystone correction circuit.

As is well known in the art, certain types of cathode ray tubes now in general use for conelectrc signals entail the production of pictures having a keystone pattern instead of the desired rectangular pattern. This so-called keystone effect results from the fact that the mosaic of the tube, in order to be supported perpendicularly to the optical system which focuses the picture to be transmitted, must be disposed at an angle to the axis of the electron gun. Thus, even though the beam is deected horizontally through the same angle for each line the mosaic, it actually traces shorter lines toward the bottom of the mosaic than at the top since the lower part of the mosaic is at a lesser distance from the electron gun.

Keystone distortion may be corrected so that a scanning pattern is produced which is of substantially rectangular configuration by modulating the horizontal scanning wave in accordance with the instantaneous amplitude of, the eld Thus, as a field is traced, the field scanning wave modulates the horizontal scanning wave to gradually alter the length of successive horizontal scanning traces so that the keystone e'ect is compensated for and a substantially rectangular field is scanned.

While certain scanning systems of the prior art have accomplished keystone correction to a certain degree, they have failed to produce a rectangular scanning patternwherein all scanning lines are of precisely the same length and/or entirely free from lengthwise displacement. As a result, in such systems the picture fields reproduced at the television receiver have had ragged vertical sides o r the lower portions thereof have included lines of excessive length and/or lines which were displaced lengthwise. 'Ihe scanning fields of these systems have been particularly 4distorted at their lower edges. These forms of residual distortion have been found to be the result of modulation distortion and of the failure of the systems to remove the higher order frequency components of the eld scanning frequency from the modulated horizontal scanning wave. It is possible to eliminate the fundamental of the eld scanning frequency from horizontal scana keystone correction the cathode ray tube.

Y method of balancing ning voltages, but-prior to the present invention no apparatus has been provided for eliminating the higher order of frequency components of the field scanning frequency as well as the fundamental thereof.

Accordingly, the primary object of this invention is to provide a novel method and apparatus for obtaining keystone correction in television systems and for eliminating the distortion in such systems which is effected by keystone correction.

Another object of this invention is to provide a novel modulation circuit in connection with the horizontal scanning circuit of a television system for correcting keystone distortion, such modulation circuit to require only a low voltage power supply.

A further object of this invention is to provide circuit particularly adapted for use in connection with electromagnetic scanning systems in cathode ray apparatus. y

A still further object of this invention is to provide a method and apparatus for eliminating higher order distortions of the scanning elds in television systems by reducing the potentials to ground in the output circuit of the horizontal scanning control circuit.

There is provided in accordance with this invention, a horizontal scanning control circuit comprising a scanning generator for generating simultaneous line scanning waves of equal amplitude but of opposite phase. simultaneously impressed on the input of a balanced modulator wherein their amplitude is modulated in accordance with the instantaneous amplitude of each eld scanning wave. There is provided in the cathode circuit of the balanced modulator, a choke coil wherein field scanning fundamental frequencies together with the higher A order components thereof are balanced to eliminate them. The modulated line scanning waves are added in the balancing choke coil and impressed on the line scanning deection coils of The balancing choke coil is connected at its mid point to ground through a reactance which has an extremely low value at the frequency involved and therefore, substantially no potential appears in the output circuit having frequencies corresponding to the fundamental field scanning frequency or its higher order components. The particular circuit and the various components of the field scanning wave lto obtain the modulated horizontal scanning Wave, provides a picture eld having substantially perpendicular sides and also sides of straight line form so that there is no These waves are choke coil 35, and the anodes of which are con.; 65

eilitsible distortion of the rectangular scanning For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

Referring to the drawing:

Fig. 1 is a circuit diagram illustrating the preferred embodiment of the invention;

Fig. 2 is a diagram illustrating keystone distortion ,together with the desired rectangular scanning field;

Fig. 3 is a diagram illustrating one type of distortion occurring in conventional keystone corrected scanning elds; and

Fig. 4 is a diagram illustrating distortion occurring in conventional keystone corrected scanning iields as a result of the presence in the horizontal scanning wave of the eld scanning Wave.

In Fig. 1 of the drawing, there is illustrated a circuit for generating line scanning waves and a modulating circuit for introducing keystone correction, together with a cathode ray tube for transmitting electrical signals corresponding to an optical image. There is provided a conventional timer chain I for generating synchronizing pulses to control the horizontal synchronizing signal generator 2 and the eld scanning signal generator 3. The output .chronizing signal generator 2 comprises a series of square-top pulses having a frequency depending upon the number of lines it is desired to scan. It may be assumed for purposes of illustration that the image will b'e scanned at the rate of 30 frames per second. The horizontal scanning will be of the order of 525 lines fore, the horizontal scanning frequency will be of the order of 15,750 cycles per second.

The horizontal synchronizing pulse vwave is converted into a saw-tooth scanning wave by the saw-tooth wave generating circuit 5. 'This circuit includes a rst time-constant circuit comprising a condenser 1 and a resistor 8 which are connected in the anode circuit of the tube I0. A second time-constant circuit comprising connected with the horizontal synchronizing gen,-

erator 2 by means of condenser II and grid resistor I4. The grid of tube I is normally biased to render it nonconductive.

The output of generator 5 is impressed on the conductors IB and I1 which are connecte to the input circuit of a modulator through coupling condensers 22 and 23 Modulator 20 includes a pair of similarv thermionic tubes 25 and 25, the cathodes of which are connected to a balanced nected to the output of field scanning generator through conductor 66. Choke coil comprises a pair of matched windin 31 together with a tapbetween the two windings which is connected to ground through condenser 28 and resistor 29 in parallel. The choke coil 35 is shunted by resistor 3l. coupled through a condenser to the output circuit of the balanced modulator 20 for converting the horizontal scanning wave into a current wave of the desired amplitude. 'y I of higher order components A power ampliner 38 is 4I is supported. at an angle to the hereinbefore, the cathode ray beam normally coupled with per frame and, therey duce appreciable would scan a eld having a keystone configuration as distinguished from the desired rectangular shape. In accordance with the present invention, however, the output of amplier 38 is deecting coils 42 of 'thetube 40 through transformer 39. Transformer 39 should have such characteristics that it will not introdistortion in the horizontal scanning wave having a fundamental frequency of the order of 15,750 cycles. The timer chain I v impulses to the neld scanning generator 3 wherein saw-tooth voltages are generated in synchronism with the timer chain output` for controlling the eld scanning of mosaic 'M vThis control is accomplished by iield scanning coils 45. The ileld scanning voltage waves are also impressed on-the anodes of modulator tubes 25 and 26 through conductor 46 connected as shown in Fig. 1. The anode polarizing poten- V tial for tubes 25 and 26 is also fed from any suit- 30 able anode circuit of field scanning generator 3 through conductor 45. 'l 4 In order to transmit electrical signals representative of an optical image, an optical system, drawing, focuses the image on a cathode ray beam gennot shown in the mosaic I whereupon erated by the electron trace successive fields windings t2 and 45.`

The line scanning of scanning lines by the is controlled initially from series of timing pulses. Horizontal synchronizing generator 2, controlled by the timer chain I, develops a series of square-top pulses, as illustrated in Fig. 1, the frequency of these pulses being of the order of 15,750 cycles per second.

Assuming the generator 5 to have reached its stable operation and to be in lsuch condition that its output voltage wave is at its zero point, at this time the tube I0 is biased to cut-off and conditions are such that condenser 1 is gradually charging fr m anode supply B+ through resistor 8 and the voltage across this condenser therefore is gradually increasing in amplitude. At the same time condenser I2 is discharging at a gradual rate through resistor I3. The rate of charge and discharge ofthese condensers depends, of course. upon the time-constants of their respective circuits. Since time-constant circuits 1, 8 and I2, I3 are identical, the voltages across condensers 1 and' I2 will be respectively increasing and de- This change Contooth waves of equal amplitude and opposite polarlty.v These saw-tooth waves mentalV frequency controlled by generator 2, and therefore, are of 15,750 cycles per second.

The sa -tooth voltagedare applied across regun of tube 40 is caused'to output of which consists of a are of a fundai synchronizing l the order'of l Hence, there appears I2 simultaneous sawl i l 1 l l i i l i l i l i the mosaic 4 l lar, as shown in scanning waves are not modulated to correct their asoman sistors 61 and B8 of modulator 2B Vand are therefore, impressed on the grids of tubes and 26 and since they are of opposite polarity and of similar shape and amplitude, the inputs of tubes 25 and 26 are balanced. The output of the eld scanning generator 3 is impressed on the anodes of tubes 25 and 26 in parallel by way of conductor 46, and therefore, the horizontal scanning waves are modulated to have gradually increasing amplitudes in accordance with the increasing amplitudes of the eld scanning waves during each field scanning period. The output currents of tubes 25 and 26 consist of the line scanning waves in opposite polarity together with eld scanning wave components in phase with vone anothert Hence, in" the opposing windings 36 and 31 of the choke 35, the line scanning waves are added while the eld scanning wave components thereof oppose each other and are cancelled out. Not only is the fundamental cf the field scanning Wave cancelled out but also the higher order frequency components thereof, the coke having exactly the same number of closely coupled turns in each of its windings 36 and 31. The use of the balancing choke coil 35 provides a single ended output for modulator 20, and therefore, the modulated lne scanning wave may be coupled through condenser 44 to power amplifier 38. Power amplier 38 converts the output of modulator 20 into a saw-tooth current wave of the desired amplitude. The output of amplifier as is translated by the transformer 39 and impressed across deiiecting windings 42, under the influence of which the cathode ray beam of Picture signals are derived from mosaic M in the manner Well known in the art. As pointed out hereinbefore, resistor 29 and condenser 28 are of such values that the impedance to ground from the center tap of choke coil 35 is extremely low at the frequency of 15,750 cycles. Therefore, the potential across choke coil 35 is substantially entirely representative of the modulated horizontal scanning Wave. Thus, there may not exist any appreciable higher order com. ponents of the field scanning frequency in the output of the modulator. arises from this arrangement in that a low voltage power supply is adequate. Since the horlzontal scanning waves, modulated'during each eld, are of increasing'amplitude as the beam scans successive lines from the top to the bottom of the mosaic, the scanning eld is rectangu- Fig. 2 at a, b, c and d. If the amplitude, the eld scanned by the beam in tube 40 would have a keystone shape such as indicated by e, f, g and h in Fig. 2. Furthermore, the pre cisely balanced saw-toothl wave generator and modulator circuit, shown in Fig. 1, provides a straight .edge scanning eld and prevents the scanning iield from having an irregular form,

such' as shown in Figs. 3 and 4 wherein certain lines, as scanned in the mosaic 4|, are lengthened or displaced horizontally as a result of modulator unbalance.

The balanced choke coil 35, as provided in accordance with the present invention, is particularly adapted to eliminate the higher order frequency components of the eld scanning frequency which appear in the modulated line scanning wave. As described hereinbefore, the coil 35 is wound in such a manner that the turns of each of the windings 36 and 31 lie side by side and. adjaent'to one another throughout their length.

y closely coupled tube 40 is deflected across A further advantage Thus, there is extremely close inductive coupling betweenwindings- 3B and 31 and minimum leakage reactance therebetween. If there were a substantial leakage reactance, it would appear as an impedance in series with winding-36 and this would result in a voltage drop across it which would be impressed on coupling condenser 44. Therefore, the higher order frequency components of the eld scanning wave in the modulated line scanning waves would create a distortion voltage, corresponding to the higher order components, across the input circuit of power ampliier 38,

However, the provision of the closely coupled choke coil'35 substantially eliminates the leakage reactance effect and also provides a substantially perfectly balanced output circuit across modulator 20. Thus, there can exist no appreciable distortion in the output of the modulator.

If a coupling element such as a center-tapped transformer` were connected across the anode is provided in Therefore, the transformer would introduce a substantial leakmodulator 20 and consequently an appreciable higher order frequency voltage in that'circuit which would introduce distortion in the input circuit of amplifier 38 traced by the cathode actance occurs.

lThe various circuit elements of the generator and modulator shown in Fig. 1 have been indicated as having certain constants for purposes of illustrating a workable form of the invention, but it is not intended that the invention should be necessary.

While there has been described what is at present considered the preferred embodiment of the invention, it will those skilled in the art that various changes and modiications may scope of the invention.'

I claim:

, 1. In a television system, a keystone correction circuit 'comprising a pair of and distortions would ooo-ur in the seid `scanned by the cathode ray of tube 40.

4 ulating the amplitude of lthe high-frequency voltages in accordance with the variation in amplitude of said low-frequency' saw-tooth voltage, a balanced choke' coil comprising a windings each of which is connected to one of the cathodes osaid devices whereby low-frequency components of said modulated voltages are balanced out, and a pair of denection windings coupled across said choke coil.

2. In combination, a pair of vacuum discharge devices connected in push-pull relation and including input circuits, anodes, cathodes and grids, a source of high-frequency pulses in the input circuits of said devices adapted to produce. simultaneous saw-tooth voltages of equal amplitudes and opposite polarities on the vices, a source of low-frequency saw-tooth voltage connected to the anodes of said devices formodulating the high-frequency voltages, and a balanced choke coil connected between the cathodes-of said devices to balance out the low-frequency components of said modulated voltage.

3. In combination, a pair of vacuum discharge devices and including input circuits, anodes and grids of said decathodes, a. source of high-frequency pulses cou. pled to the input circuits of said devices, a source of low-frequency pulses connected to the anodes of said devices for modulating the high-frequency pulses, a balanced choke coil connected between the cathodes of said devices to balance out the low-frequency components of said modulated voltage, and a load circuit coil.

4. In combination, a pair of vacuum discharge devices connected in push-pull relation and including input circuits, anodes and cathodes, a

source of high-frequency pulses coupled to the frequency pulses,

input circuits of said devices, a source of lowfrequency saw-tooth'voltages connected to the anodes of said devices for modulating the highmeans connected between the cathodes of said devices to balance out the lowfrequency components and their higher order components in said modulated voltages and a cathode ray deection means coupled to said meansq l CLYDE E. HALIMARK.

coupled to said choke 

